The use of highly-purified recombinant HIV-1 gp41 protein 566 and HIV-2 gp35 protein 996 in development of an immunodiagnostic test to distinguish between HIV-1 and HIV-2 infections was previously reported. A simple, inexpensive and sensitive immuno-dot blot test was developed that utilizes a very small quantity of these highly-immunoreactive HIV antigens, and requires less serum for reac-tivity than standard Western blot techniques. In addition, the previously developed HTLV-I recombinant env protein, rpBI, and HTLV-II recombinant env protein, rpBII, in differential immunoblot analyses was utilized, in conjunction with the polymerase chain reaction (PCR) technique, to isolate HTLV type II-related viruses from New World monkeys, namely, the spider monkey. A virus was isolated that showed both immunological cross-reactivity and PCR amplification patterns suggestive of a simian T-lymphotropic virus (STLV) type II. No STLV-II virus has, so far, been isolated. Further genetic, biochemical and immunological analyses are necessary to characterize this virus as an STLV-II isolate.